Playing a piano should be a joy, and nothing can cause you to wince like a key that is sticking. A stuck key can throw off your playing and make it difficult to finish a piano piece properly. Locating and fixing problems within your piano isn't hard, as long as you know what to look for. Sticking keys are usually the result of a sticky hammer inside the piano and are often caused by wear and tear or humidity.
Open the cover to your piano's interior. Run your hand over the keys (or play scales) to see which hammer is sticking.
Take canned air and blow out the hammer's mechanism. Sometimes the smallest item -- mouse droppings, grit -- will stop a hammer from working properly.
Spray the hammer's mechanisms with the piano lubricant. If it still will not respond, then talk to a piano technician. Often the cause of a sticking hammer will be in the piano's more complicated actions that are difficult to gain access to and are best left to a professional.
Do not use WD-40, machine oil or silicone sprays to free up sticking components. These sprays might solve the problem temporarily, but they will become sticky over time and require a special treatment by a piano technician to remove.